The present invention relates to an insulating plug for an electric discharge lamp apparatus having a structure in which a lamp-side connector, to which a connector for supplying electric power can be connected and disconnected, is integrally provided for the rear end of the main body of an insulating plug having a front end to which an arc tube is secured and held.
As shown in FIG. 15, a conventional electric discharge lamp apparatus has a structure that an arc tube 3 is, through a metal lead support 2a and a ceramic disc 2b, integrally secured to the front end of an insulating plug body (an insulating base) 1 made of synthetic resin.
Note that a hole, through which a rear end of the arc tube is inserted, is formed in the front surface of the insulating plug body 1.
The arc tube 3 has a structure that an ultraviolet-ray shielding globe 5 is, by welding, integrated with an arc tube body 4 having an enclosed glass bulb 4a which is a light emitting portion in which electrodes 4b are disposed opposite to each other. Thus, the enclosed glass bulb 4a is surrounded and hermetically sealed by the ultraviolet-ray shielding globe 5. The electrodes 4b are connected to lead wires 4e extending from the arc tube 3 through molybdenum foil members 4d bonded to pinch seal portions 4c.
On the other hand, a cylindrical portion 1a extending rearward is formed at the rear end of the insulating plug body 1. A belt-shape negative-side terminal 6 electrically connected to a front-end lead wire of the arc tube 3 through a lead support 2a is exposed on the outer surface of the cylindrical portion 1a. In the central portion of the cylindrical portion 1a, a positive-side terminal 7 electrically connected to a rear-end lead wire of the arc tube 3 is exposed. The foregoing elements (the cylindrical portion 1a, the terminal 6 and the terminal 7) constitute a lamp-side connector C2 to which a connector C1 for supplying electric power can be connected and disconnected.
A ceramic insulating sleeve 2c is fitted to a lead support 2a forwards projecting over the insulating plug body 1. Thus, insulation between a lead support 2a which is a positive-side passage for electric power and a lead wire at the rear end of the arc tube 3 is maintained, the rear-end lead wire being a positive-side passage for electric power.
As shown in FIG. 16, the insulating plug body 1 is formed by injection molding such that the front end surface having a relatively large thickness of the insulating plug body 1 faces a gate 9 of the molds 8a and 8b. A slide mold 8c is provided for forming a recessed groove for receiving a negative-side terminal 6 which is formed into an undercut portion.
However, the conventional insulating plug 1 has a problem in that hollow portions B (hereinafter called "voids") which cause dielectric breakdown are formed at the base portion of the rear-end cylindrical portion 1a.
The inventors of the present invention have conducted investigations, resulting in the following fact being detected. Resin injected through the gate 9 is introduced from the front-end cylindrical portion into the rear-end cylindrical portion 1a so that the cylindrical portion 1a is filled with the resin. In the rear-end cylindrical portion 1a which is apart from the gate 9 and which has a large thickness, resin cannot sufficiently be supplied. Moreover, the molding pressure in the foregoing portion is made to be lower than that in the portions in the vicinity of the gate 9. Therefore, gas cannot sometimes sufficiently be removed. The resin injected into the cavity is solidified such that the peripheral portion which is brought into contact with the mold is first solidified. In the thick wall portions, contraction and solidification occur slowly as compared with thin wall portions when the molding process is performed. Therefore, resin is pulled by the peripheral portion which has first been contracted and solidified and, therefore, deformation takes place. Thus, an estimation is made that voids B are formed.
Therefore, the inventors have formed an inner cylindrical portion and a cylindrical portion for surrounding the inner cylindrical portion which are provided for the front end portion of the insulating plug. Thus, the thickness of the overall portion from the front end portion of the insulating plug body to the base portion is reduced as compared with that of the conventional structure so that a uniform thickness is realized. In addition, the extending end surface of the rear-end cylindrical portion 1a having the largest thickness in the insulating plug body 1 is caused to face the gate 9. As a result, no void was observed in the overall portion of the insulating plug body 1 including the inside portion of the rear-end cylindrical portion 1a and the front-end cylindrical portion (the inner cylindrical portion and the outer cylindrical portion).
The present invention is established to solve the above-mentioned problems experienced with the conventional technique and on the basis of the facts detected by the inventors.